Large scale integrated circuit may also be referred to as “LSI”) modules generate a large amount of heat when reduced in size because the degree of integration and the operation speed are dramatically increased. In general, therefore, the LSI modules are cooled by using the forced cooling means.
As the forced cooling means, various cooling means such as the air cooling system and the water cooling system have been developed. As cooling means which is the most excellent in cooling capability among those forced cooling means, a liquid cooling system in which LSI modules are immersed in a liquid coolant and the liquid coolant is circulated is developed.
A technique of a heat generation element mounting semiconductor apparatus having a cooling liquid hermetically sealing case which serves as a shielding member mounted on a printed-circuit board is disclosed in, for example, Patent Document 1 (Japanese Patent Publication No. 3424717). In this heat generating elements mounting semiconductor apparatus, heat generating elements (for example, LSI modules) are accommodated in a cooling liquid hermetically sealing case, and the cooling liquid is circulated along a predetermined flow path by a partition wall provided in the cooling liquid hermetically sealing case to cool the heat generating elements.
According to this technique, efficient cooling can be conducted even if the mounting density of heat generating elements is increased.
As the capacity of data transmission becomes large, optical transmission systems are being researched, developed, and applied to practical use. In the above-described optical transmission system, optical elements (for example, light emitting elements and light receiving elements) which convert optical signals and electric signals to each other and optical transmission means (for example, optical waveguides and optical cables) which transmit optical signals are used. The optical elements are different from conventional electronic components which handle only electric signals. In other words, each of the optical elements includes external connection terminals to input and output electric signals and external optical signal connection means to input and output optical signals.
Various techniques for embedding optical transmission means in a substrate on which optical elements and electronic components are mounted mixedly are being developed.
A technique of a mixed optical and electrical wiring board having an optical fiber embedded layer obtained by embedding an optical fiber in a partial layer of an electric wiring board having an electric circuit mounted thereon is disclosed, for example, in Patent Document 2(Japanese Patent Application Laid-open No. 10-126018). A technique of an optical waveguide formed in a substrate is disclosed in Patent Document 3(Japanese Patent Application Laid-open No. 7-114049) and Patent Document 4(Japanese Patent Application Laid-open No. 6-27334). According to these techniques, optical transmission means such as optical fibers and optical waveguides can be embedded in a printed-circuit board.
A printed-circuit board having optical transmission means embedded therein proposed in the conventional art will now be described with reference to the drawing.
FIGS. 5(a) and 5(b) are schematic diagrams showing a printed-circuit board having optical transmission means embedded therein proposed according to a conventional art. FIG. 5(a) is a perspective view of a printed-circuit board having optical cables embedded therein, and FIG. 5(b) is a perspective view of a printed-circuit board having optical waveguides embedded therein.
In FIG. 5(a), an LSI module 91, a connector 98 and an optical element 92a are mounted on a printed-circuit board 90a. In addition, a (flat) land 93 described in Patent Document 1 is formed so as to correspond to a seal face shape of a shield case (not illustrated). An optical cable 95 connected at each end to an optical connector 94 is embedded in the printed-circuit board 90a. One end of the optical cable 95 is arranged from inside of the annular land 93 to the optical element 92a. The other end of the optical cable 95 is arranged from outside of the annular land 93 to the external of the printed-circuit board 90a. 
Although not illustrated, in the printed-circuit board 90a, a shield case serving as a shielding member to be mounted on the printed-circuit board is attached onto the land 93 via an O ring. The optical connector 94 located inside the land 93 is connected to the optical element 92a. 
Because of this arrangement, a liquid coolant (for example, FLUORINERT (registered trademark of the 3M Corporation) which is an inactive liquid) flows in the hermetically sealed shield case. As a result, the LSI module 91 is cooled effectively. Optical signals can be transmitted between the inside and the outside of the shield case via the optical cable 95 embedded in the printed-circuit board 90a. 
In FIG. 5(b), a printed-circuit board 90b differs from the printed-circuit board 90a in that an optical waveguide 96 is embedded instead of the optical cable 95. Other structures of the printed-circuit board 90b are substantially the same as those of the printed-circuit board 90a. 
One end of the optical waveguide 96 is located inside the annular land 93 and the other end of the optical waveguide 96 is located outside the land 93. One end of the optical waveguide 96 is connected to an optical element 92b via an opening (not illustrated) formed in the printed-circuit board 90b. The other end of the optical waveguide 96 is connected to an optical element (not illustrated) via an opening 97. Because of this arrangement, optical signals can be transmitted between the inside and outside of the shield case via the optical waveguide 96 embedded in the printed-circuit board 90b. 